Pest Management

What is Pest Management?

A pest species can be any species that humans consider undesirable. Any organism that reduces the availability, quality, or value of a human resource can be classified as a pest. This designation in no way reflects the organism’s role in the natural ecosystem but is more an indicator that they are in conflict with humans. Plant pests, also referred to as weeds, are included in the discussion of Non-native Invasive Plant Removal.

A pest in one area may not be considered a pest elsewhere. Often organisms rise to pest status because they escape normal control by natural regulating agents. This is achieved through direct or indirect importation to a new region or by human activities which reduce or eliminate the efficiency of their natural enemies. Without controls on population growth, organisms can rapidly achieve levels at which damage is caused thus becoming pests (e.g., locust swarms stripping landscapes bare). However, organisms do not need to exist in large numbers to be a pest. For example, the codling moth (Cydia pomonella) does not lay many eggs compared to many insects and often produces only one generation each year (Begon et al. 1996). However, because it blemishes apples, making them commercially undesirable, the codling moth is considered an important agricultural pest.

Pest management is therefore a means to reduce pest numbers to an acceptable threshold. An acceptable threshold, in most cases, refers to an economically justifiable threshold where application of pest control measures reduces pest numbers to a level below which additional applications would not be profitable (i.e., where additional costs of control exceed additional benefits). Pest eradication (i.e., complete removal) is usually not a viable option.

Methods of control can be categorized as chemical, biological, cultural, physical/mechanical, or genetic, and are discussed in further detail below.

  • Chemical. Chemicals (e.g., insecticides, herbicides, rodenticides) can be broad-spectrum (non-selective) or narrow-spectrum (selective), and can be organic or inorganic. Chemicals used to regulate pest abundance can act as nerve toxins (for insects and mammals) and growth regulators/inhibitors. Chemicals can also be used to affect pest abundance through more indirect means, such as releasing pheromones to disrupt breeding behavior and interfere with mating. Chemical pesticides are often toxic to non-target organisms including the pest’s natural enemies, can persist in the environment affecting water supply, soil productivity, and air quality, and can be biomagnified in the food chain. Inappropriate use of pesticides can result in target pest resurgence from killing off natural enemies, secondary pest outbreaks by removing natural enemies of other organisms and allowing them to rise to pest status, and evolved resistance to the pesticide.
  • Biological. Due to any number of reasons, including those mentioned in the Chemical and Cultural sections, compromising the effectiveness of natural enemies often allows potential pest organisms to experience virtually unregulated population growth and enables them to reach pest status. Biological control involves the use of a pest’s natural enemies (e.g., predators, pathogens, parasites and parasitoids), to control pest abundance. Measures to conserve or enhance the impact of natural enemies should be attempted first. Perhaps biological control is most known for importation of natural enemies, often from the pest’s area of origin, to control non-native pests (e.g., importing vedalia bettles to control cottony cushion scales which were attacking California citrus orchards). A number of safeguards are necessary before implementing importation actions to ensure imported organisms will not pose additional threats to non-target organisms. A third approach to biological control involves augmenting natural enemies through rearing and periodic releases and can be inoculative (natural enemies are released early in the season) or innundative (natural enemies are released as a biological pesticide).
  • Cultural. The effectiveness of natural enemies can be compromised by human practices. Application of broad-spectrum pesticides which kill off natural enemies in addition to target pest species, the type of crop plant, the crop environment, and cropping practices. Modern crop varieties often inadvertently create conditions which favor pest species (e.g., pest species which have bored deeper into larger fruit making them inaccessible to natural enemies). Crops are often monocultures, consisting of a single crop species, which creates a homogenous habitat often lacking key requirements of natural enemies, thus favoring pest species. Moreover, many harvesting practices prevent natural enemies from persisting in annual crops. Examples of cultural practices that encourage natural enemies and discourage pest persistence include intercropping (multiple crops in the same field) to make it more difficult for pests to find a host plant, planting trap crops which attract pests away from harvest crops and which can later be treated with select application of pesticides, and delaying planting times to coincide with times where pests have emerged and died off for the season.
  • Physical. Manual or mechanical removal, or installation of physical barriers can be used to exclude pest species. Removal methods include use of animal traps, sticky cards for insects, manual removal of insects from plants (e.g., hand picking or spraying with a hose), removing diseased or infected materials (e.g., pruning branches or removing diseased litter). Physical barriers such as fences, nets, mulch, and tree trunk guards can exclude pests and reduce the damage they inflict.
  • Genetic. Genetic alteration to reduce pest impacts is not as widely known or publicly available as other control options. Autocide is one type of genetic control and involves using the pest itself to induce increased mortality rates. Sterile males are introduced into the population, which, after mating with females, creates infertile eggs. This is an expensive option with many limitations including potential for reduced competitive viability of the introduced sterile males versus naturally occurring fertile males. Straightforward genetic manipulation to create pest resistant plant strains is another form of controlling pest impacts. However, genetic manipulation research and development is costly, and introduces a whole other series of ethical and environmental issues that are not easily addressed. Genetic manipulation is not a viable control option for the general public.

Integrated Pest Management (IPM) is an increasingly popular process for controlling pests. IPM considers the ecosystem as a whole and takes into consideration a balanced mix of the aforementioned control methods to produce the most effective and least damaging plan. All the methods are mutually augmentative with chemical control means as the last resort in the plan. Ideally, an IPM plan would result in a sustainable system without need for much costly follow-up maintenance.

A number of insects and pathogens have been identified as pests particularly for their impacts to vineyards, orchards, and other agricultural industries important to Napa County’s economy. The following table presents a number of important pest species in Napa County as well as their deleterious effects.

Pest Type Effects
Pierce’s Disease (Xylella fastidiosa) Fungus A lethal disease of the grapevine which is often spread by insects with piercing/sucking mouthparts that feed on xylem sap such as the glassy-winged or blue-green sharpshooter.
Oak Root Fungus (Armillaria mellea) Fungus Parasitizes the roots of orchard trees, oaks and many other woody plants including grapevines sometimes causing sudden death.
Botrytis Bunch Rot (Botrytis cinerea) Fungus Directly affects grape bunches reducing yield and quality of grapes. Infected fruit causes off-flavors and aromas in wine, and is unsuitable for most wine production.
Powdery Mildew (Uncinula necator) Fungus Damages or destroys berries, reduces photosynthesis and can affect wine flavor and quality.
Grape Leafhopper (Erythroneura elegantula) Insect Damage vines by inserting their stylets to suck out the contents of leaf cells reducing the vine’s photosynthetic ability, and are often a nuisance to vineyard workers during harvest
Spider Mites (Tetranychus pacificus,and Eotetranychus willamettei) Insect Destroys grapevine leaf tissue feeding on the lower surface of the leaf reducing photosynthesis and other physiological functions.
Pocket Gopher (Thomomys bottae) Mammal Damages vines by chewing on roots while tunneling underground, and above-ground portions as well. Gophers cause other problems by chewomg on plastic irrigation pipes, and building burrows which can divert and concentrate runoff, causing significant erosion problems in the vineyard as well as lawns and gardens.
Starling (Sturnus vulgaris) Bird Starlings, in addition to a number of birds, including house finches, and robins, flock consume ripening and mature grapes causing significant damage. In Napa County, flocks of starlings can reach numbers in the thousands, and damage from them can be the most severe.
Deer (Genus odocoileus) Mammal Damages vines by grazing on the foliage, often stripping canes clean of leaves causing extensive damage and stunting of vines.